Java代码中的奇怪逻辑错误

Question

interface New<T> {
   boolean func(T n, T v);
}

class MyFunc<T>{
T val;
MyFunc(T val){
   this.val = val;
}
void Set(T val){
   this.val = val;
}
T Get(){
   return val;
}
boolean isEqual(MyFunc<T> o){
   if(val == o.val) return true;
return false;
}

public class Main {
  static <T> boolean check(New<T> n , T a, T b){
     return n.func(a,b);
  }
  public static void main(String args[]){
   int a = 321;
   MyFunc<Integer> f1 = new MyFunc<Integer>(a);
   MyFunc<Integer> f2 = new MyFunc<Integer>(a);

   boolean result;
   //f2.Set(f1.Get()); //if i uncomment this line result become true
   System.out.println(f1.val + "  " + f2.val);
   System.out.println();

   result = check(MyFunc::isEqual, f1, f2);
   System.out.println("f1 isEqual to f2: " + result);
  }
}

Why result is false when 'a' is used in both f1 and f2 ? Why result is true when f2.Set(f1.Get()); is uncommented? Please explain me where I am making mistake.

Answer 1

In the .isEquals() method you're comparing tho wrapper objects with the == operator, which compares the objects references if those are not within the Integer internal cache.

Since 321 is out of the Integer cache, the == operator returns false , because the references are different ( f1 refers to different memory address than f2 ).

When you explicitly set the references to be equal (with f2.Set(f1.Get()) ), then it's not surprising the program outputs true .

If you set a to something between -128 and 127 , the program will output true .

Answer 2

a is auto boxed to Integer and for int a = 321; you will have two different Objects for f1 and f2 .

FOR EXAMPLE

Integer a1 = new Integer(321);
Integer a2 = new Integer(321);

and for a1 == a2 you will have false because == compare reference instead of value you should use equals method to check value in your isEqual method.